1. Field of the Invention
This invention relates to instantaneous floating point (IFP) amplifiers which amplify a fluctuating input signal to a level within preselected limits.
2. Description of the Prior Art
In seismic exploration, sound waves are commonly used to probe the earth's crust as a means of determining the types and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing sound waves through that medium. In the reflection seismic method, sound waves or impulses are generated at a transmission point at or near the earth's surface, and sound waves reflected from subsurface reflecting boundaries are received at one or more receiving points. The received waves are detected by seismic detectors, e.g., geophones, which generate electrical signals at their outputs. Information relating to subsurface formations is contained in these signals, and they are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
In a typical seismic field recording system, the seismic detectors are arranged in arrays or nests. The outputs of these arrays are time-division multiplexed, amplified, digitized, and recorded. In a typical time-division multiplexed system, the output of each array is sampled one per sample time, and it is common for the sample time to be either one millisecond, two milliseconds, or four milliseconds.
The amplification of the output of a seismic detector array is complicated in that the amplitude of the input sound waves is a function of time. Additionally, the amplitude of the reflected sound waves decreases with time, because recording is typically continued after the generation of input sound waves is terminated. Accordingly, it is undesirable in seismic operations to utilize an amplifier having a fixed gain.
The amplifier which has commonly been utilized to amplify the output of a seismic array is known as an instantaneous floating point amplifier. The gain of this type of amplifier varies depending upon the magnitude of the input signal, and the amplifier is usually designed to apply a gain to this input such that the amplifier output, when sampled, is at a level within preselected limits. Typically, the amplifier is designed to amplify the input signal to a level between one-half and the full scale output of the amplifier.
A typical instantaneous floating point amplifier includes a plurality of cascaded amplifier stages, and the number of stages and the gain of each stage determine the maximum gain that the amplifier can apply to the signal presented at its input. A given stage of the amplifier may be used or may be selectively bypassed, depending upon the amount of gain that must be applied to the input signal to amplify it within the preselected limits.
A typical instantaneous floating point amplifier also includes control circuitry which determines, for each input signal, those stages of amplification which are required to amplify the input signal to within the preselected limits, and which stages are to be selectively bypassed. This control circuitry typically generates a gain word which is representative of which stages of the amplifier are presently being utilized to amplify the input signal. Of course, as the number of stages of amplification in the amplifier increases in an amplifier using this selective bypassing approach, the generation of the gain word becomes more complex. It is, therefore, desirable to minimize the number of stages of amplification which are required to implement a given amplifier by using an approach other than the selective bypassing of stages.
Another problem with prior art IFP amplifiers using the selective bypassing approach is that the higher gain stages almost always go to saturation. This necessitates a longer interval for the operational amplifiers in those stages to settle before the next sample signal is received, and it also complicates offset voltage control since the offset voltages of the IFP's operational amplifiers change when they are saturated. Therefore, it is desirable to provide for an IFP amplifier whose stages do not become saturated.
Instantaneous floating point amplifiers, like amplifiers in general, have inherent offset voltages which are amplified together with the input signals. Since it is imperative that the ultimately recorded data be as uncorrupted by noise as possible, this amplified offset voltage must be removed before later processing begins. This technique of removing the offset voltage has commonly been referred to as "nulling the amplifier".
Prior art amplifier systems typically correct the offset voltages of the system's operation amplifiers on a component-by-component basis, or use a nulling technique which disables the amplifier for offset correction while data is being received. Component-by-component offset correction requires a great deal of hardware and complex circuitry while the latter, disabling technique results in a loss of valuable data. Therefore, it is desirable to achieve offset voltage correction using less hardware without any loss of valuable data.
Gain control in prior art IFP amplifiers is achieved in several ways. A typical prior art device achieves gain control by first measuring the sample signal and applying a low gain to the sample signal which is of such a magnitude that the IFP amplifier cannot go into saturation. The resulting signal is again measured, and a higher gain is then applied, again so that the applied gain will not cause the IFP amplifier to go into saturation. This process is repeated until the appropriate gain is finally determined.
It is apparent that the above process is very time-consuming and requires that a lot of measurements be taken for each sample. Thus, it is desirable to achieve gain control by taking a single measurement and applying the appropriate gain in a single step. The use of a bipolar logarithmic amplifier like that in the present invention accomplishes this result.
The instantaneous floating point amplifier of the present invention overcomes these and other shortcomings of the prior art in the manner described below.